Field of the Invention
The invention relates to an aqueous coagulatable polymer dispersion comprising at least one polymer dispersed in an aqueous phase, thermoplastic microspheres containing a blowing agent (propellant), and at least one additional component selected from the group consisting of polyols, polyamines and thermoplastic polymers.
Further, the invention relates to a coagulate obtainable by thermal and/or mechanical and/or ultrasound-initiated coagulation of the aqueous polymer dispersion according to the invention, an adhesive or binder comprising or consisting of the coagulate, a substrate entirely or partially coated with the coagulate, a process for producing such a coated substrate, and coated substrates obtainable by such a process. The coagulate may be used as an adhesive or as a binder in 3D-printing procedures.
Description of Related Art
Folding cardboard containers and boxes as well as paper bags and bags made from sheet composites are versatile packaging types employed in all fields of product transport or product presentation. The bonding/adhesion steps necessary for preparing the packaging use different systems (dispersion and hotmelt adhesives) having different advantages and disadvantages and take place at different places of the supply chain.
For example, the manufacturer of packaging materials produces the corrugated board (testliner, kraftliner) from paper and cardboard grades. From this raw material, the packaging blank is prepared by printing, laminating, painting and punching processes. While for tray applications (tray construction) the work of the manufacturer is complete at this stage, folding and telescope boxes require further that a preliminary bonding of the longitudinal seam be additionally effected. In the majority of cases, this is achieved through the use of dispersion adhesives. The produced packagings are delivered to the packer who then places the goods to be transported (i.e., to be marketed) into the packaging and seals the packaging subsequently through use of an adhesive bond.
In order to ensure high throughput numbers and high productivity, hotmelt adhesives based on ethylene-vinyl acetate (EVA) and polyolefins (PO) are mostly employed for sealing cardboard cartons and folding boxes. Since adhesives based on these raw material have setting times of from less than one second up to three seconds, they are ideally suited to quickly stem the restoring forces of the packaging that occur after the initial pressing. A dispersion adhesive is usually incapable of achieving this. The quality of the adhesives increases from EVA to PO.
While commodity products based on EVA have a lower stability in the melt, lower adhesion properties with respect to different surfaces, a tendency to smell offensive and low transparency, products based on POs mostly do not exhibit these drawbacks. Products based on POs are, however, more expensive.
Disadvantages found in each of the abovementioned systems include, for example, lower heat resistance (max. 70° C.) in comparison to dispersions, a weakening of the bond caused by migrating components from the packaged goods (tea packagings), or lower mechanical strength (blow filling). Thus, so-called combination bonding in which a hotmelt adhesive is applied together with a dispersion is preferred in the field of packaging bags. The hotmelt adhesive provides an initial cohesion of the packaging in the manufacturing process, while the long-term stability and durability is effected by the dispersion.
The use of hotmelt adhesives in the packaging industry also involves drawbacks in terms of processing. Thus, the hotmelt adhesive must be melted before use and maintained at a processing temperature over the time of use. The hose lines and nozzle-application system are also heated. This results in substantial energy consumption. Although this energy consumption has already been reduced through developments of the application device manufacturers (Nordson Freedom® and Liberty® System), there is still potential for improvement, as, for example, a material with a high tendency to blocking cannot be promoted by such systems.
One approach to solving this problem is described in WO 2011/072237. Therein, a thermally activatable adhesive composition of the plastisol type is described, which contains particles of a first polymer and particles of a second polymer dispersed in a liquid organic carrier medium (e.g., vegetable oils, epoxidized vegetable oils, bio-diesel, glycerol). When a particular activation temperature is reached, the particles of the second polymer either dissolve or plasticize in the carrier medium.
The production of hardcover books remains, despite competition from electronic media, an important market with high demands on the finished product.
In book production, a distinction is made between softcover and hardcover books. Depending on the type of the respective product, different bonding steps are necessary for the production. One of the most important steps is adhesive binding, i.e., the connection of the individual sheets of the book with each other to form the so-called body of the book and, in the case of a softcover book, also with the cover. In terms of bonding technology, this process step is, depending on the kind of machine used, effected by means of dispersion adhesives, conventional thermoplastic hotmelt adhesives based on EVA or PO, and increasingly commonly with reactive polyurethane (PUR) hotmelt adhesives. Each of these systems has both advantages and disadvantages. In dispersion adhesives, the good penetration into the sheet edge (fiber) and the accompanying strength as well as the lay-flat behavior are valued. However, the slow production speed is criticized by many bookbinders. This can be increased, for example, by using hotmelt adhesives. However, lower strength and worse lay-flat behavior must often be accepted instead.
In this application case, an adhesive system is desirable that combines the advantages of the individual adhesive systems without exhibiting their drawbacks. Filter media, for example, those for the automobile or interior fields, have the important function of protecting high-performance aggregates, such as engines or electronic components, from contamination caused by soot, dust or other pollutants. Filter materials made from treated and untreated grades of paper and various non-wovens or nanofibers are employed in this regard. In the industrial fabrication of the filter media, adhesives are employed for a variety of purposes. In addition to the bonding of adsorbants such as active charcoal, the adhesive primarily serves the constructive and shape-providing bonding of the filter. In this regard, two fabrication steps are of particular importance. In the so-called pleating method, the two-dimensional filter material is folded and the individual folds bonded together at an exact pre-defined distance. The object of pleating is to maximize the filter surface within the available space. During the fabrication, an adhesive strip is thus placed on the filter composite before the folding. In the subsequent folding, adhesive strips come to rest on top of one another, thus ensuring a consistent distance. In addition, another adhesive strip is applied to the folded parts, which provides for additional strength. As a last step of fabrication, the pleated material is embedded in a frame. The latter provides for shape stability and strength even under the highest stress.
Today, thermoplastic hotmelt adhesives based on EVA and PO are predominantly employed as the adhesive system. The demands on these systems in view of loadability are very high. Aside from an open time appropriate for the processing process (for example, in order to enable pleating), the setting must occur very quickly. In addition, finished filter materials are in some cases subjected to a high temperature treatment after the adhesive bonding, which the bond must of course resist. Especially in view of this stress, the use of dispersion adhesives with a high heat resistance would be advantageous. In addition, the dispersion achieves a better penetration into the filter medium, which additionally increases the mechanical strength. This would be an additional advantage for frame bonding. Since dispersion systems have significantly slower setting speeds in comparison to hotmelt adhesives, the use of dispersions results in a deterioration of productivity. Therefore, it would again be desirable in this case to have an adhesive system available that combines the advantages of the individual adhesive types without having their respective drawbacks.
From the point of view of users of adhesives, the use of dispersion and hotmelt systems are associated with the abovementioned advantages and disadvantages in the described exemplary applications. Therefore, it was the object of the invention to develop a composition and a process that make use of the respective advantages of the individual systems in respect of transport, processing and final strength but without exhibiting the respective disadvantages.
Therefore, it is desirable to provide an aqueous polymer dispersion that can be formulated, filled and transported like a dispersion adhesive, but can be triggered in an application device (for example, by temperature, shear, ultrasound or pressure) to behave with regards to its properties (setting speed, strength build) like a hotmelt adhesive and which, after application, additionally retains the properties of a dispersion (heat resistance, flexibility, resistance to migrating substances).
The production method of so-called 3D-printing is increasingly gaining importance as a key technology. The advantages of this technology include a high freedom of object design and short times from idea conception to marketing. By way of example, one of the main applications of this technology is therefore in the field of “rapid prototyping”. Markets into which this production technology has already entered include, for example, the automobile, aviation and health industries. Furthermore, 3D-printers increasing making inroads into the home-user market in respect of the fabrication of replacement parts or self-designed, “ready-to-use out of the 3D-printer” objects.
In principle, the production of the 3D-printed object from plastic materials can be effected in several ways (3Druck.com). These include, for example:
1.) Additive Layer Manufacturing
                This form of manufacturing requires a powdery solid as a base material and a binder or fixing agent. Through a printing head similar to that of an ink-jet printer, the liquid binder is applied with pinpoint accuracy to the powder layer then cured. This is followed by an additional application of the powder to the bound first layer and a further fixation. In this way, a 3D-object is formed layer-by-layer from individual 2D-layers, which are bonded to one another.2.) Fused Filament Fabrication        This manufacturing method makes use of meltable plastics as base materials. Using a printing head (extruder) similar to that of a hotmelt application device, the pre-melted polymer is applied to a heatable platform layer by layer. This can be done on the one hand by moving the printing head, or the platform. After the previous layer has cooled and hardened, the application of the next layer can be effected.3.) Liquid Materials (e.g., Stereolithography)        The production is based on a tank filled with a liquid medium. The surface of the liquid is cured and hardened layer-by-layer by introduced UV radiation, a laser, an electron beam or the like. Then, the object is lowered, so that new liquid is constantly available for construction at the surface. Because of the possibility of pinpoint laser control, this method is one of the most precise, but also one of the most expensive.        
Different raw materials are used in 3D-printing depending on the method employed. Thus, for example, two-component binders based on diamines and dicarbonyl compounds are employed for the fixing of powders (e.g., PMMA or PA) in additive layer manufacturing (DE 10 2010 056 346 A1). After mixing, they react to form Schiff bases.
In fused filament fabrication technology, thermoplastic materials such as PLA, PA or polycarbonates are used. They are often obtainable in the form of a wound coil, which is directly inserted into the device.
In the case of stereolithography, reactive monomers that are linked together by the selected energy source (activation) are employed.
With respect to the binders employed, each of these methods exhibits both advantages and disadvantages. In the case of the two-part mixture in the field of additive layer manufacturing, the requirement of the precise mixing of the two components on the powder material may be mentioned as a possible disadvantage. In the field of fused filament fabrication, the polymers employed must first be melted at high temperatures. In addition, the selection of possible polymers is (still) very limited. Although the storage in the form of a coil offers ready accessibility, this is at the expense of a higher space requirement. In stereolithography, the reactive mixture must be well protected or stabilized in order to prevent a possible premature reaction.
Therefore, it is further desirable to provide an alternative binder that extends the range of employable polymers and minimizes drawbacks as far as possible, which can be formulated, transported and stored as an aqueous binder, thus having many of the handling advantages of a dispersion, but behaving like a meltable binder upon the action of a trigger in an application device (for example, temperature, shear, ultrasound or pressure), and which has the properties of a conventional binder after application (heat resistance, resistance to various substances, flexibility, mechanical strength and the like).